Myofibrils, the workhorses of skeletal muscle, consist of interdigitating thick and thin filaments, their surrounding membrane systems and the proteins associated with each. Myosin Binding Protein C (MyBP- C), a thick filament associated protein, is believed to play a role in the structural stability of the sarcomere and modulation of contraction. A novel variant of MyBP-C slow, variant 1, localizes to the M-band where it surrounds the myofibril, unlike other MyBP-C isoforms which are found within the cross-bridge containing C- zone of the A-band. Through its interaction with obscurin, a giant sarcomeric associated protein, 1 hypothesize that variant 1 plays a role in the organization and stabilization of thick filaments, M-bands and their surrounding sarcoplasmic reticulum (SR) membranes through a scaffolding apparatus that both proteins are involved in at the M-band. To test this hypothesis 1 have proposed two specific aims: 1.) To characterize the spatial and temporal expression pattern of MyBP-C slow variant 1 in skeletal muscle during development and at maturity; 2.) To investigate the morphological role of MyBP-C slow variant 1 in the organization of thick filaments and surrounding SR membranes and its functional influence on SR Ca2+ cycling. For aim 1 I will specifically assess the developmental expression profile of variant 1 during in vivo mouse embryogenesis and during de novo myofibrilogenesis. Furthermore, I will characterize the localization of variant 1 at the ultrastructural level and with respect to specific fiber types. In aim 2 I will use siRNA technology to reduce the levels of variant 1 using in vivo whole muscle electroporation and subsequently monitor the morphological and functional effects on the sarcomere. Knowledge gained from the proposed studies will greatly advance our understanding of the molecular mechanisms that underlie the assembly and stability of thick filaments and M-bands and the anchoring of the SR membranes to the nearby contractile apparatus. This research as it relates to public health is a necessary contribution to the study of muscle disease. Mutations in the cardiac MyBP-C gene have been identified as the cause of hypertrophic and dilated cardiomyopathies. It is easy to speculate that the same mutations in the cardiac gene, if translated to the skeletal muscle isoforms will cause similar structural defects resulting in distinct skeletal myopathies and muscular dystrophies.